The present invention relates to an optical soliton pulse generator for high-speed fiber optic communication.
With the recent progress of optical amplification techniques, the fiber optic communication technology is now advancing toward ultra-long distance communication and it is becoming a reality to implement fiber optic communication across the Pacific Ocean without using any regenerative repeaters. With conventional transmission systems, however, an increase in the transmission rate allows the influence of deterioration of the transmission characteristic to grow which is based on the wavelength dispersion characteristic and nonlinear optical effect of the optical fiber, imposing limitations on the high-speed, large-capacity transmission. In recent years an optical soliton communication system has been in the limelight as a system which surmounts the limitations on the speeding up of transmission owing to the wavelength dispersion characteristic and the nonlinear optical effect. The optical soliton communication system is a system that makes positive use of wavelength dispersion characteristic and nonlinear optical effect of the optical fiber which are contributing factors to the deterioration of the transmission characteristics of prior art systems and that transmits optical short pulses intact while balancing the broadening of pulses by the wavelength dispersion in the optical fiber and the compression of pulses based on the nonlinear optical effect. A time multiplex system and a wavelength multiplex system are also relatively easy to implement and are suitable for high-speed and large-capacity transmission. Heretofore, a pulse light source for the optical soliton communication has been implemented by a semiconductor mode locked laser with an external cavity or a gain switched semiconductor laser.
To realize the soliton communication utilizing the optical fiber, an optical soliton pulse generating technique is of importance. The conditions for the optical pulse are that its temporal waveform is in the shape of a squared hyperbolic secant (sech.sup.2 t, where t is time) and that its frequency spectrum does not excessively broaden and is expressed by a Fourier transformation of the temporal waveform (a Fourier transform limit). That is, it is necessary that the product of the full width at half maximum of the temporal waveform and the full width at half maximum of the frequency spectrum be 0.315.
To suppress interference between adjacent optical pulses which occurs on the optical fiber transmission line, it is desirable that the pulse width be 20% of or less than the pulse interval.
Incidentally, in an experimental optical soliton communication system at present, optical pulses are used when the product of the full width at half maximum of the temporal waveform and the full width at half maximum of the frequency spectrum is within 0.441 or so.
A semiconductor mode locked laser with an external cavity, which is a typical example of conventional optical pulse generators, has a construction in which one end face of the laser diode is given an anti-reflection coating, a reflector is disposed in front of the said one end face and the other end face of the laser diode and the reflector constitute an optical resonator. Since the semiconductor laser is driven by a sinusoidaly modulated current signal synchronized with the light round trip time of the optical resonator, the modulation rate is fixed to the length of the optical resonator and the product of the full width at half maximum of the temporal waveform and the full width at half maximum of the frequency spectrum exceeds 0.5. Moreover, this method presents a problem in its long-term stability as it employs the optical resonator which is susceptible to changes in environmental conditions, for example, a temperature change or mechanical vibration.
On the other hand, according to the gain switching method which drives the semiconductor laser directly with short current pulses, the modulation rate is not fixed in principle, but in practice, it is hard to change the modulation rate arbitrarily, because it is inevitable to use a resonator type microwave circuit such as a comb generator. With this method, the frequency spectrum excessively broadens more than in the case of employing the semiconductor mode locked laser, and the product of the full width at half maximum of the temporal waveform and the full width at half maximum of the frequency spectrum becomes greater than 1. Consequently, no soliton optical pulses can be obtained in this case; hence, it is necessary to use a new technique for suppressing the excess spectral brodening, such as a narrow-band optical filter.
In either case, since the oscillation wavelength of the laser undergoes substantial changes with its direct modulation, the spectrum of pulses broadens excessively and they cannot be used intact as optical pulses for the soliton communication. Furthermore, it is difficult to implement an optical soliton pulse generator which is stable for a long period of time.